A CCD imager constructed according to well known techniques comprises a matrix of many cells, each responsive to visible light for generating a number of electrons based upon the intensity of light impinging on the cells. The electrons collected within each cell of the matrix are then shifted to an output, and serialized, to produce an analog signal which corresponds to a portion of the image. The analog signals can then be amplified and re-transmitted to a CRT or other type of television screen. Typically, CRT displays take the serial signals and by raster-scan techniques, regenerate the image on the phosphor screen of the CRT.
CCD imagers are typically utilized in television cameras and other types of picture processing equipment to function as a transducer for converting visible images into corresponding electrical signals. CCD imagers adapted for such use are described in detail in U.S. Pat. No. 4,229,752, assigned to Texas Instruments Incorporated. The light energy reflected from an object is converted into an electrical image in the silicon material of the CCD integrated circuit. A CCD imager typically comprises a matrix of several thousand individual silicon cells which are exposed to the light reflected from the object. The photons which are characteristic of the reflected visible light enter the silicon cells and generate a number of electron-hole pairs in correspondence to the light intensity. Thus, for light rays having a high intensity, a larger number of electron-hole pairs will be generated than light rays having a low intensity. Each cell of the CCD array is isolated from the adjacent cells so that the generated electrons within the cells remain associated with each respective cell. In this manner, an electrical image representative of the object image is thus captured. The substrate of the silicon CCD imager is biased to remove the holes of the electron-hole pairs, thereby leaving the captured electrons as the charge which is representative of the object image.
The isolation which typically separates the CCD cells comprises a boron diffusion which extends down into the silicon material sufficiently to provide lateral isolation between the cells. Photons associated with visible light enter the silicon material and generate electron-hole pairs at a depth generally no greater than about 0.1-0.3 microns. Hence, boron diffusions to such depth are adequate to prevent the electrons captured in one cell from migrating to an adjacent cell. CCD imagers fabricated according to such techniques are generally inadequate to convert long wavelength object images or images from higher energy radiation, such as x-rays, into corresponding electrical signals, as the radiation thereof penetrates more deeply into the silicon material before electron-hole pairs are generated. Accordingly, the generated electrons do not become captured in each cell, and thus migrate laterally. This lateral migration results in a poor picture resolution, wherein the video picture tends to smear. Some attempts have been made to improve device performance by increasing the depletion region in the silicon substrate via increased substrate resistivity, however, the advantages of this approach have been limited because of the unavailability of a good lateral isolation scheme.
Attempts to form a deeper lateral isolation by driving the boron diffusion deeper into the silicon substrate provide an inadequate isolation structure, in that the diffusions also spread laterally, thus requiring significantly more wafer area per cell. Such deep isolations are therefore counterproductive with regard to attempts to increase the packing density or increase the number of CCD cells per chip. In the virtual phase type of CCD arrays, there must also be provided a conductive path between the substrate and the virtual phase electrode for proper cell operation. Thus, deep nonconductive cell isolation structures alone do not provide an adequate solution for cell isolation.
From the foregoing, it can be seen that a need exists for an improved CCD cell isolation technique which provides deeper isolation structures, without requiring additional lateral wafer area. An associated need exists for a CCD imager which is responsive to longer radiation wavelengths to provide corresponding pictorial displays of infrared radiation, x-ray radiation and other long wavelength radiations. Yet another need exists for a deep isolation cell structure and a vertical conductor to render virtual phase CCD circuits responsive to the noted types of radiation.